Sensing apparatus using frequency changes

ABSTRACT

An apparatus and method for determining various physical quantities such as temperature, pressure, stress, strain, distance and the like in a manner such that a change in the physical quantity of interest results in a measurable change in the frequency of oscillation of a signal generated within the apparatus.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.08/118,090, filed Sep. 8, 1993, now U.S. Pat. No. 5,635,919, which is acontinuation of abandoned U.S. patent application Ser. No. 07/563,510,filed Aug. 6, 1990, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to an apparatus for determining variousphysical quantities. More particularly, the invention concerns anapparatus for determining various physical quantities such astemperature, pressure, stress, strain and the like, in a manner suchthat the changes in the physical quantity of interest result in changesin the frequency of oscillation of a signal generated within theapparatus.

2. Description of the Invention

The vast majority of prior art sensing and transducing systems rely onconverting the physical quantity of interest into an analog voltage orcurrent at some stage of the signal processing. An example of this isthe strain gauges that are used as the transducer systems for measuringstrain, pressure, force and various other physical quantities.

Such systems bring with them the inherent problems of minimizing thenoise that is introduced into the signal and of determining the truesignal level in the inevitable presence of at least some noise.Overcoming these problems typically results in systems that are eitherdelicate, expensive, or both.

A limited number of other transducing systems attempt to directlymeasure the time delay or "time of flight" of a signal directed oversome particular path in order to measure a physical property ofinterest. Such systems typically are used to measure distance, as in thecase of radar, or electrical characteristic impedance, as can be donewith a time-domain reflectometer. At the most fundamental level, thesesystems depend on measuring the time delay between a transmitted and asubsequently received signal.

In these systems, the necessary accuracy for the measurement of thisdelay can be in the sub-nanosecond range, and direct measurement of suchtime intervals is a substantial technical challenge. A limited number ofother systems as exemplified by U.S. Pat. No. 4,885,433 (Schier) avoidthe direct time interval measurement and make use of the phasedifference introduced into a modulated signal by the time delay. It isof course then necessary to measure this phase difference, a processwhich is also sensitive to external influences and noise of varioussorts. Furthermore, some systems resort to converting the time delay orphase difference into a voltage by means of an appropriate circuit, andthis brings with it the problems previously mentioned.

Lastly, one small group of devices specifically converts a velocity ofinterest directly into a frequency by taking advantage of the dopplershift introduced by some moving object of interest. This group istypified by doppler radar systems and so-called "ring-around flowmeters." These devices function by transmitting a signal at a movingtarget of interest (in the case of the radar) or through a moving mediumof interest (in the case of the flow meter). Due to the motion of thetarget or the medium, the frequency of the signal is shifted, and thisfrequency is then detected by any of a variety of means. In many cases,and in particular in the case of doppler radar, an intermediate step ofhomodyne or heterodyne conversion is necessary before the frequency ofinterest can be detected.

In any device relying on a doppler shift, if the object of interest isreceding from the transmitter the signal which is subsequently detectedis lowered. Conversely, if the object is approaching the transmitter,the frequency is increased. Inevitably, such devices are adapted tomeasure only the velocity of the object of interest.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a multi-purposesensing and transducing system adaptable to sensing a wide variety ofphysical quantities. With the same basic signal processing system, thisinvention is easily adapted to measuring absolute distance, index ofrefraction, stress, strain, force, torque, temperature, pressure,magnetic field strength, electric field strength and other physicalquantities.

Another object of the invention is the conversion of the physicalquantity of interest into the frequency of a signal which can be readilymeasured. Conversion to a frequency has the specific advantage that afrequency can be measured to within one part in one million with nospecial effort or expense whatsoever, and with very moderate effort,frequency can be measured to within one part in one billion. It isgenerally accepted that the measurement of frequency is among theeasiest and most accurate measurements that can be made.

A further object of the invention in a preferred embodiment is toachieve the conversion of the quantity of interest into a frequency withno intermediate electronic processing steps, thus eliminating theadditional complexity, inaccuracy, cost, and noise inevitably associatedwith such processing.

In one embodiment, the present invention provides an apparatus formeasuring a number of different physical quantities in a manner suchthat a change in the physical quantity of interest results in a changein the frequency of oscillation of a signal generated within theapparatus. The invention relies on a feedback system which is made toself-oscillate at a frequency determined by the overall transmissiondelays in the system. The system is configured so that variation in thephysical quantity of interest causes a variation in the transmissiondelay and consequently causes a change in the frequency of oscillation.For example, to sense the temperature a signal transmission pathoccupied by gas at constant pressure might be used. In this instance, asthe temperature of the gas increases, the gas becomes less dense,lowering its index of refraction. This means that an electromagneticsignal transmitted through the gas will have a higher velocity in thegas, thus decreasing the overall transmission delay. This decrease intransmission delay results in a higher frequency of oscillation in thesystem. Another example could involve the use as a signal transmissionmedium of selected electro-optic materials whose properties change as afunction of an external electric or magnetic field. Still anotherexample could involve the use of an optical fiber whose index ofrefraction varies as a function of stress or strain applied to thefiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generally diagrammatic view of one embodiment of the sensingapparatus of the present invention.

FIG. 2 is one possible schematic diagram for the preferred embodiment ofthe invention.

FIG. 3 is a diagrammatic view of an alternate embodiment of theinvention.

FIG. 4 is a diagrammatic view of another alternate embodiment of theinvention.

FIG. 5 is a diagrammatic view of yet another alternate embodiment of theinvention.

FIG. 6 is a diagrammatic view of still another embodiment of theinvention.

DESCRIPTION OF THE INVENTION

Referring to the drawings and particularly to FIG. 1, one form of theapparatus is there shown which comprises a transmitter 12 capable ofemitting a modulated signal 14, a receiver 16 capable of receiving themodulated signal and transmission means 17 for conveying the transmittedsignal from the transmitter to the receiver. Also comprising a part ofthe apparatus of this embodiment of the invention is a signaltransmission means between the receiver 16 and the transmitter 12, whichincludes a transmission path, a transmission medium within thetransmission path and an inverter means 24 for inverting the transmittedsignal; a frequency determining means 26 for determining the frequencyof the signal present in the transmission path; and computation means 19for computing the signal delay time through the transmission means.Given the material properties of the transmitting medium, thecomputation means also functions to compute the value of the physicalquantity, as for example, stress or strain, which caused the observedtransmission delay through the transmitting medium.

With regard to the transmission means, which is identified in FIG. 1 asa transmission medium 17, this medium is selected so as to be sensitiveto the particular physical quantity desired to be sensed. Moreparticularly, the medium is chosen so that the transmission time of thetransmitted signal varies according to some type of physicalinterference. For example, an optical fiber can be made from selectedplastic materials whose index of refraction varies with applied stressor strain. If the index of refraction is made to increase, thetransmission time of the transmitted signal will also increase.Conversely, if the index of refraction is made to decrease, thetransmission time of the transmitted signal will also decrease. It is tobe understood that, depending upon the physical quantity of interest,the transmission medium can include air, various liquids and gases andnumerous other media which will appropriately convey the transmittedenergy.

Turning to FIG. 2, the transmitter 12, which can take various forms wellknown to those skilled in the art, such as a laser diode or an LED, ishere shown as a laser diode 18 capable of transmitting a modulatedsignal through the transmission medium. The receiver 16, shown here ascomprising a photo diode 20, is adapted to receive the signal 14 afterit has passed through the transmission medium. The receiver 16 willtypically have an amplifier 21 to amplify the received signal. Thesignal transmission path, identified by the numeral 22, includes theinverter means, here provided as an inverting amplifier 24. If desired,the inverting function could be included in the amplifier stage of thereceiver. However, the inverter is here shown separately for sake ofclarity. Also included within the transmission path is the frequencydetermining means which is shown in FIG. 2 as comprising a frequencymeter 26. It is to be understood that the frequency determining meanscan comprise any of a number of electronic frequency counters of acharacter well known to those skilled in the art. Operably associatedwith the frequency meter is the previously identified computation meanswhich can be provided as one of several types of readily commerciallyavailable digital computers. The interconnection and operation of thefrequency determining means and the computation means is well within theskill of the art.

The operation of the apparatus of the invention is dependent uponestablishing an oscillation in the system, the frequency of whichdepends on the external physical influences acting on the transmissionmedium which is disposed between the transmitter and receiver.

Beginning with the transmitter, assume that a signal 14, such as lightemitted from a laser diode, is being emitted. After transmission of thelight through the transmitting medium, and with the attendant delaycaused thereby, the signal is detected by receiver 16 which comprises aphotodiode 20 where it is converted to an electrical output signal. Thiselectrical output signal is, in turn, inverted and then applied to thetransmitter 12, tending to turn the transmitter off. Again, aftertransmission through the transmitting medium, the receiver detects thislower level signal and once more converts this signal into an electricalsignal. This electrical signal is in turn inverted and applied to thetransmitter, tending to turn the transmitter on. At this point, thesystem has completed one cycle of the oscillation and is in the samestate as when the oscillation began. This being the case, the systemwill commence with the next cycle of oscillation in the same manner ashas just been explained, and the oscillations will continue so long asthe apparatus remains operational.

That is, as shown in FIG. 2, diode 18 has a transistor 18a associatedtherewith, and the base of the transistor receives inverted signal 12awhich, when high, turns the transistor on, and when low tends to turnthe transistor off. Therefore, when the signal 12c outputted by thephotodiode is high, the inverter's output 12a is low, and this shutsoff, or mostly shuts off, the transistor 18a, so that the diode output12c is a zero or low level signal. The receiver then receives this lowlevel input signal 16a and outputs signal 16b, which inverter 24inverts. This high inverted signal 12a then turns the transistor on toturn the diode on high as signal 12b from a voltage source flows throughthe diode.

The time to complete one cycle of the oscillation is composed of thefollowing time intervals. The first interval is the time required forthe light signal to travel from the transmitter to the receiver throughthe transmission medium, which may be identified as do. The nextinterval is the time required for the received signal to be converted toan electrical signal and be conveyed back to the transmitter. Thisinterval may be identified as de. This time interval de is a measurableand fixed value for a given transmission circuit operating under anyparticular set of conditions. The third interval is the time requiredfor the light signal, this time a low level light signal, to once againtravel from the transmitter to the receiver which is again do. The finaltime interval is the time for the received signal to once again beconverted into an electrical signal and be conveyed back to thetransmitter, which is again de.

The four time intervals described above are the four time intervalsnecessary to complete one cycle of oscillation in the system. The totaltime to complete a cycle is then simply the sum of those four intervalsand can be expressed as

    dt=2de+2do,

and can be measured in seconds or fractions of a second.

The frequency of oscillation of the system is simply the number ofcycles completed in a second. This number is given as

    f=1/dt=1/(2de+2do).

It can be seen that this frequency is a function of the delay time, dobetween the transmitter and receiver. As shown in FIG. 1, theoscillation frequency is measured by any of a number of availablefrequency measuring devices. This being done, the transmission delaythrough the transmission medium can be determined. Furthermore, if thisdelay time do through the transmission medium is made to vary under theinfluence of some outside physical quantity--such as the effect ofstress or strain on optical fibers--it is apparent that the value of theinfluencing physical quantity can be readily calculated from thefrequency of oscillation given the material properties of thetransmission medium.

Referring to FIG. 6, the present invention can also be used as discussedabove to provide a relationship between an applied force 19 applied tothe transmission medium 17 and a characteristic of the medium. Forexample, where the characteristic of the medium is known for response toa physical force, an unknown physical force can be determined. Thetransmitter 12 transmits a modulated continuous signal 14 from thetransmitter through the transmission medium. The unknown force 19applied to the transmission medium 17 changes the speed at which themodulated signal 14 passes through the transmission medium to create atime-shifted modulated signal 15. The time-shifted modulated signal 15is received by the receiver 16, and is then processed in a processingmeans such as an inverter 24. The processed signal 21 is fed back to thetransmitter to create the modulated continuous signal 14 emitted fromthe transmitter. The frequency of an envelope of the modulatedcontinuous signal is measured by frequency sensing means 26 and themeasured frequency is passed to the computation means 27. Within thecomputation means the measured frequency is correlated with knownmaterial properties of the transmission medium to determine the externalinfluence or unknown force 19 (such as a physical force, stress, strain,temperature, electricity, magnetism, etc.) applied to the transmissionmedium.

As previously mentioned, various optical fibers can be used as stressand strain sensitive transmission media. For sensing other physicalproperties, other media would be selected. For example, to sensetemperature, a transmission medium comprising a gas at constant pressuremight be used. As the temperature increases, the gas would become lessdense, lowering its index of refraction and increasing the frequency ofoscillation within the system. Other examples include the use of variouselectro-optic materials whose properties change as a function of anexternal electric or magnetic field. Specifically, the index ofrefraction of lithiumniobate is sensitive to electric fields, and assuch it could be used as the transmission medium in a sensor of electricfield strength. The invention may also be used to determine an unknownindex of refraction of a material or an unknown electro-optic effect onthe index of refraction by applying known voltages to the material andusing the same relationship of the frequency of the system to theapplied external influence, determining the unknown index. This methodand apparatus could be applied to determine an unknown index forresponse to known temperatures, or an unknown index for response topressure, stress, strain, etc. The structure of the invention would bethe same, except in the calculation means the index of interest isunknown and the external influence is known.

Before considering alternate forms of the invention, it is important tounderstand that, as opposed to other devices such as doppler radar orring-around flow meters, the signals generated in the device of thepresent invention do not fundamentally depend on the velocity of anypart of the sensing system or any other object of interest. At the mostbasic level, the device develops its output signal in response to atransmission delay between a receiver and transmitter in combinationwith any other delays in the system. As such, for example, when used fordistance measurements, the apparatus of the invention develops an outputsignal that represents the distance to an object of interest regardlessof whether the object is in motion or is stationary.

In contrast, the prior art doppler radar and ring-around flow meters andother similar systems develop an output signal that is fundamentallydependent on the velocity of the object of interest. Consequently, adoppler radar system will have the same output when measuring astationary object within its range regardless of the distance to thatobject. It therefore does not directly measure the distance to theobject.

Similarly, a ring-around flow meter develops its output signal only inresponse to the velocity of the medium of interest. If the medium isstationary, the output signal does not change.

FIG. 3 shows an alternate embodiment of the apparatus of the inventionwhich is similar in many respects to the apparatus shown in FIGS. 1 and2 and like numerals have been used to identify like components. In thissecond form of the invention the transmission path includes a reflectingelement 28. As described previously, the frequency of oscillation of thesystem will still be determined by the transmission media in thetransmitted and reflected paths. If so desired, however, the length ofthis transmission path and its attendant delay can now be carried bymoving the reflector 28 nearer to or farther from the transmitter 12 andreceiver 16, as well as by moving the transmitter and receiverthemselves. It should be noted that the transmission media shown in FIG.3 could be, but is not necessarily the same material.

FIG. 4 shows another embodiment of the invention which is also similarto the embodiment shown in FIG. 3, but with the reflector replaced by areceiver-transmitter pair 30. In the embodiment of the invention shownin FIG. 4, wherein like numbers are used to identify like components,the transmitted signal is received by receiver 2 (32) after a firsttransmission through an appropriate transmission medium. The signal isthen retransmitted by transmitter 2 (34) again through a transmissionmedium before being received by the main receiver. The auxiliaryreceiver and transmitter function much as does the reflector in FIG. 3to relay the transmitted signal back to the receiver, and, as in theprevious case, the frequency of oscillation of the system can also bevaried by moving the auxiliary receiver and transmitter relative to therest of the system.

In the embodiments of the invention shown in FIGS. 3 and 4 the frequencymeasurement and distance computation are carried out in a manner similarto that previously described, with details of the computationappropriate to the geometry and function of the system being taken underconsideration.

It is to be understood that the signals need not be limited to opticaland electrical signals. Other forms of radiation in the electromagneticspectrum, or acoustic radiation, or any other type of controllablesignal source can be used in the system in which the essential featureis that a feedback path is provided which causes the system to oscillatebased on the transmission delay between a transmitting element and areceiving element.

Before considering the apparatus of the invention shown in FIG. 5, it isimportant to observe that the apparatus of the invention shown in FIGS.1 through 4, and as described in the preceding paragraphs, can also beused as a position determining apparatus. For example, assume that thetransmission medium identified in FIG. 1 is air and that the transmitter12 is adapted to emit a signal 14 such as light from a laser diode.After transmission of the signal 14 across an intervening space with theattendant transmission delay, the signal is detected by receiver 16,where it is converted to an electrical signal. This electrical signal isin turn inverted and then applied to the transmitter 12, tending to turnthe transmitter off. After transmission across the intervening space,the receiver again detects this lower level signal and once moreconverts this signal to an electrical signal. This electrical signal isin turn inverted and applied to the transmitter, tending to turn thetransmitter on. At this point, the system, as before, has completed onecomplete cycle of the oscillation and is in the same state as when theoscillation began. This being the case, the system will commence withthe next cycle of oscillation in the same manner as has just beenexplained, and the oscillations will continue.

The time to complete one cycle of the oscillation is composed of thefollowing time intervals. The first interval is the time required forthe light signal to travel along the transmission path from thetransmitter to the receiver, and is denoted as do. The next interval isthe time required for the received signal to be converted to anelectrical signal and be conveyed back to the transmitter, and isdenoted as de. This time interval de is a measurable and fixed value fora given transmission circuit operating under any particular set ofconditions. The third interval is the time required for the lightsignal, this time a low level light signal, to once again travel fromthe transmitter to the receiver which is again do. The final timeinterval is the time for the received signal to once again be convertedto an electrical signal and conveyed back to the transmitter, which isagain de.

The four time intervals described above are the four time intervalsnecessary to complete once cycle of the oscillation in the system. Thetotal time to complete a cycle is then simply the sum of those fourintervals and can be expressed as

    d=2de+2do,

and can be measured in seconds or fractions of a second.

As previously discussed, the frequency of oscillation of the system issimply the number of cycles completed in a second. This number is givenas

    f=1/dt=1/(2de+2do).

It can be seen that this frequency is a function of the delay time, do,between the transmitter and receiver. Specifically, if the delay time isincreased by widening the separation between the transmitter andreceiver, the frequency will decrease as dictated by the equation above.Conversely, if the delay time is decreased by bringing the receiver andtransmitter closer together, the frequency will increase.

By measuring the oscillation frequency using frequency meter 26, thetransmission delay between the transmitter and receiver can bedetermined, and hence the distance between the transmitter and receivercan be readily computed using appropriate computation means.

Turning now to FIG. 5, another form of position determining apparatus ofthe present invention is shown. This form of the invention comprises astylus shaped transmitter 36 and two spaced apart signal receivingmeans, or receivers 38 and 40. Transmitter 36 can be selectivelyconnected to receivers 38 and 40 by a switching means shown here as astandard multiplexer 42.

In using the apparatus of FIG. 5, when the transmitter 36 is connectedto receiver 38, the system will oscillate at a frequency that isrepresentative of the distance between transmitter 36 and receiver 38.By using the techniques already explained, this distance can readily bedetermined. In a similar fashion, when the transmitter 36 is connectedto receiver 40, the distance between the transmitter and receiver 40 canbe determined. With these two distances now known, computation means 27acan use any conventional method of triangulation computation todetermine the position of the transmitter relative to the two receivers.

It should be noted that in this latest described embodiment of theinvention, the position of the receivers and the transmitter may beinterchanged, and the system will still function substantially aspreviously described. In such an arrangement, the stylus shaped housingin FIG. 5 would contain a receiver instead of a transmitter, and twotransmitters would be substituted for the two receivers. The multiplexerwould still connect an appropriate receiver and transmitter pair so thatthe corresponding distance measurement could be made. Much as describedbefore, with two distance measurements.

As a further illustration, if two transmitters (receivers) and threereceivers (transmitters) were used, the position and tilt of the lineconnecting the two transmitters (receivers) could be determined from thesix available distance measurements.

Additional receivers and transmitters may also be added in order to makethe system more robust in the face of obstructions, electricalinterference and "noise" in general.

Having now described the invention in detail in accordance with therequirements of the patent statutes, those skilled in this art will haveno difficulty in making changes and modifications in the individualparts or their relative assembly in order to meet specific requirementsor conditions. Such changes and modifications may be made withoutdeparting from the scope and spirit of the invention, as set forth inthe following claims.

What I claim is:
 1. A method of determining an unknown externalinfluence applied to a transmission medium, the method comprising thesteps of:transmitting a modulated continuous signal from a transmitterthrough the transmission medium; applying an unknown external influenceto the transmission medium which changes the speed at which themodulated signal passes through the transmission medium to create atime-shift resulting in a time-shifted modulated signal; receiving thetime-shifted modulated signal and demodulating it to form a time-shifteddemodulated signal; processing the time-shifted demodulated signal andfeeding it back to the transmitter to mix it with the continuous signalfor modulating the continuous signal and causing the continuous signalto oscillate at a frequency related to the time shift for converting thetime-shift into the frequency of oscillation, wherein the modulatedsignal has an envelope corresponding to the frequency of oscillationcaused by the processing; measuring the frequency of oscillation bymeasuring a frequency of the envelope of the modulated continuoussignal; and determining the external influence applied to thetransmission medium on the basis of the measured frequency of theenvelope and a known material property of the transmission medium.
 2. Amethod as claimed in claim 1, wherein the step of processing comprises astep for inverting the time-shifted demodulated signal.
 3. A method asclaimed in claim 1, wherein the step of transmitting is performed usingan optical fiber to transmit the modulated signal.
 4. A method asclaimed in claim 1, wherein in the step of transmitting, the transmittermeans comprises a laser diode which transmits the modulated signal, andthe receiver means comprises a photodiode which receives thetime-shifted modulated signal.
 5. A method according to claim 1 whereinthe external influence comprises an unknown physical force.
 6. Anapparatus for determining an unknown external influence applied to atransmission medium, the apparatus comprising:means for transmitting amodulated continuous signal through the transmission medium to produce atime-shift, corresponding to the applied external influence, resultingin a time-shifted modulated signal; means for receiving the time-shiftedmodulated signal and demodulating it to form a demodulated signal, meansfor processing the time-shifted demodulated signal and feeding it backto the means for transmitting to modulate the modulated continuoussignal to oscillate at a frequency related to the time-shift, whereinthe modulated signal has an envelope corresponding to the frequency ofoscillation caused by the processing means; means for measuring thefrequency by measuring the envelope of the modulated continuous signal;and means for determining the unknown external influence on the basis ofthe measured frequency and a known material property of the transmissionmedium.
 7. An apparatus as claimed in claim 6, wherein the means forprocessing comprises means for inverting the output signal from thereceiver means.
 8. An apparatus as claimed in claim 6, wherein thetransmission medium comprises an optical fiber.
 9. An apparatus asclaimed in claim 6, wherein the transmitter means comprises a laserdiode, and the receiver means comprises a photodiode.
 10. An apparatusas claimed in claim 6 wherein the external influence comprises anunknown physical force.